Brightness
brit'-nes: Used by the King James Version in Heb 1:3 for "effulgence of his glory," as in the Revised Version (British and American) and the American Standard Revised Version. The Greek apaugasma may mean either "reflection" or "radiation." Patristic usage favors the latter; compare The Wisdom of Solomon 7:26; also the Nicene Creed: "Light of Light," i.e. the Son not only manifests the Father, but is of the same substance. "What emanates from light, must have the nature of light" (Delitzsch).
